A non-reciprocal device such as an isolator or circulator is incorporated, for example, in a mobile wireless communication device such as a mobile telephone set. A typical non-reciprocal device comprises magnetic components such as a gyromagnetic component formed from a soft ferrite body and center conductors and a permanent magnet, and electric components such as a matching capacitor and a terminal resistance, all housed in a magnetic-metal case functioning as a yoke.
The center conductors are combined with the soft ferrite body, and to the assembly is applied a DC magnetic field by the permanent magnet. Each of the center conductors has one end placed on one surface of the soft ferrite body to be connected to the metal case serving as a grounding part to be grounded, and the other end arranged on the other surface of the soft ferrite body such that the conductors are insulated from each other and intersect with each other on the other surface of the soft ferrite body with a specified angle between adjacent conductors. The other end of each center conductor serves as a terminal for external connection.
Demand for the compaction of a non-reciprocal device as described above is limitless because the smaller a non-reciprocal device is, the higher marketability it can command. In response to this demand, attempts have been made with efforts mainly directed to the compaction and thinning of the soft ferrite body. Indeed, the soft ferrite body currently available comes to have a size as small as a square with a side of 4 mm or less. On the other hand, users of mobile telephone sets are increasing, and dual band sets are being brought to the market. These tendencies require not only the broadening of frequency bands applicable to this type of wireless communication, but also the broadening of frequency bands treated by a non-reciprocal device incorporated in a mobile telephone set used for such wireless communication. However, if the soft ferrite body is made small beyond a certain limit, the center conductor will have a too small inductance which will narrow the frequency band treatable otherwise compensated. If the inductance of the center conductor is too small, it is necessary, in order to achieve impedance matching, to increase the intensity of a magnetic field applied to the center conductor which in turn requires the soft ferrite body to be thickened.
As seen from above, compaction of a soft ferrite body is in opposite relation with broadening of the frequency band treatable by a non-reciprocal device incorporating the soft ferrite body. Proposals have been offered for broadening the frequency band treatable by a non-reciprocal device, and they include, for example, attachment of an impedance modifying circuit or resonant circuit to the non-reciprocal device.
Specifically, reissued patent publication WO 00/59065 discloses a technique for broadening the frequency band of a matching network by attaching an impedance modifying circuit to the input/output terminal of a lumped reciprocal element. The same invention further discloses a process including producing a laminated ceramic body by low-temperature simultaneous firing which represents an inductance and electrostatic capacitance of the impedance modifying circuit, and a load capacitance inherent to the lumped reciprocal element, and introducing the laminated ceramic body into a non-reciprocal device.
JP, H11-239009, A discloses a technique for broadening the frequency band of a non-reciprocal device with a single peak characteristic curve by modifying the element in such a manner as to allow it to have a dual peak characteristic curve. Specifically, the technique consists of connecting a resonant circuit to a grounding terminal of the non-reciprocal device, and the invention also discloses a circuit board on which the resonant circuit is mounted together with the non-reciprocal device and other circuit components.
In JP, S56-20322, A and JP, S56-24815, A, a scheme for broadening the frequency band treatable by a non-reciprocal device is disclosed that consists of attaching an element comprising an inductor and capacitor connected in series to a terminal of the non-reciprocal device, and inserting another capacitor between the connection point of the inductor and capacitor, and ground.
Broadening the frequency band treatable by a non-reciprocal device by attaching an impedance modifying circuit or resonant circuit to the non-reciprocal device described above, however, requires the deliberate introduction of an electrostatic capacitance in addition to a matching capacitance inherent to the non-reciprocal device. Introduction of an impedance modifying circuit or resonant circuit into a non-reciprocal device does not reduce the capacitance required for the inherent matching capacitor. Thus, addition of an impedance modifying circuit or resonant circuit requires the institution of an additional space in the non-reciprocal device for accepting the additional circuit, which may seriously interfere with the attempt to reduce the size or thickness of the non-reciprocal device.